1. Field of the Invention
The present invention relates to a process for producing a lithium-containing composite oxide for a positive electrode for a lithium secondary battery, which has a large volume capacity density, high safety, an excellent durability for charge and discharge cycles and low temperature characteristics, a positive electrode for a lithium secondary battery containing the produced lithium-containing composite oxide, and a lithium secondary battery.
2. Discussion of Background
Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a positive electrode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Among them, a lithium secondary battery using a lithium-containing composite oxide (LiCoO2) as a positive electrode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can operate at a high voltage at a level of 4V, whereby it has been widely used as a battery having a high energy density.
However, in the case of the non-aqueous type secondary battery using LiCoO2 as a positive electrode active material, further improvement of the capacity density per unit volume of a positive electrode layer and the safety, has been desired. On the other hand, there has been a problem of deterioration of the cyclic properties such as gradual reduction of the battery discharge capacity due to repetitive charge and discharge cycles, a problem of the weight capacity density or substantial reduction of the discharge capacity at a low temperature.
In order to solve part of these problems, it has been proposed in JP-A-6-243897 that the average particle size of LiCoO2 as a positive electrode active material, be from 3 to 9 μm, the volume occupied by a group of particles having a particle size of from 3 to 15 μm, be at least 75% of the total volume, and the intensity ratio of the diffraction peaks at 2θ=about 19° and 2θ=45° as measured by means of X-ray diffraction using CuKα as a radiation source, be of a specific value, so that it becomes an active material excellent in coating properties, self-discharge properties and cyclic properties. Further, in the publication, it has been preferably proposed that LiCoO2 does not substantially have such a particle size of smaller than 1 μm or larger than 25 μm. With such positive electrode active material, the coating properties and the cyclic properties have been improved, but, the safety, the volume capacity density and the weight capacity density, have not yet been fully satisfactory.
Further, in order to solve the problem related to the battery characteristics, JP-A-3-201368 proposes to replace 5 to 35% of Co atoms with W, Mn, Ta, Ti or Nb to improve the cyclic properties. Further, JP-A-10-312805 proposes to use hexagonal LiCoO2 as a positive electrode active material to improve the cyclic properties, wherein the c axis length of the lattice constants is at most 14.051 Å, and the crystallite size of (110) direction of crystallites is from 45 to 100 nm.
Further, JP-A-10-72219 proposes that a lithium composite oxide of the formula LixNi1-mNmO2 (wherein 0<x<1.1, 0≦m≦1), whereby the primary particles are plate-like or columnar, the ratio of (volume standard cumulative 95% size−volume standard cumulative 5% size)/(volume standard cumulative 5% size) is at most 3, and further, the average particle size is from 1 to 50 μm, has a high initial discharge capacity per weight and further is excellent in the charge and discharge cyclic durability.
Further, JP-A-2002-60225 proposes to lithiate a cobalt compound powder in the form of secondary particles having an average particle size of 0.5 to 30 μm formed by agglomeration of primary particles of cobalt hydroxide, cobalt oxyhydroxide or cobalt oxide with an average particle size of from 0.01 to 2 μm. However, also in this case, it is not possible to obtain a positive electrode material having a high volume capacity density, and further, the material is insufficient also with respect to the cyclic properties, the safety or the large current discharge properties.
As described above, in the prior art, with respect to a lithium secondary battery employing a lithium composite oxide as a positive electrode active material, it has not yet been possible to obtain one which sufficiently satisfies all of the volume capacity density, the safety, the coating uniformity, the cyclic properties and further the low temperature characteristics.